IZYUM, Ukraine – Gunfire erupted in Ukraine on Friday as military and Interior forces launched a “large-scale operation” before dawn near the edges of rebel-besieged Sloviansk and Kramatorsk in the country’s embattled eastern region to force separatists to free hostages, lay down weapons and free seized government buildings, authorities said.

Two Ukrainian helicopters were shot down by pro-Kremlin rebels and their pilots were killed, the country’s Defense Ministry said. Vyacheslav Ponomaryov, the insurgent-appointed “people’s mayor” of Sloviansk, was quoted by Russia’s Interfax news agency also as saying two helicopters had been shot down.

Meanwhile, Russia news outlets reported that a third helicopter was shot down by Kremlin-backed militia fighters. The third helicopter could not be confirmed.

The pro-Kremlin forces said at least one Ukrainian helicopter pilot was killed during the fighting, while another had been detained, according to theAssociated Press.

Ukrainian Interior Minister Arsen Avakov said in a post on Facebook that other military personnel had been wounded after the pro-Russian militants fired anti-aircraft missiles at Ukrainian military helicopters. Avakov said that nine illegal rebel checkpoints had also been destroyed, and that professional mercenaries were operating within the rebel group.